1. Field of the Invention
The present invention relates to a method used in a wireless communication system, and more particularly, to a method of handling an emergency bearer service for a mobile station in a wireless communication system.
2. Description of the Prior Art
A GSM (global system for mobile communication) system, initiated by the third generation partnership project (3GPP), is now being regarded as anew radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the GSM system, an UTRAN (universal terrestrial radio access network, UTRAN) includes a plurality of base stations and communicates with a plurality of mobile stations (MSs), also referred as to user equipments (UEs). In GSM system, a network node, e.g. a Serving GPRS Support Node (SGSN), is responsible for the delivery of data packets to the mobile devices back and forth within its geographical service area, including packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions.
A General packet radio service (GPRS) is a packet oriented mobile data service on GSM cellular communication systems. GPRS enhances the second generation (2G) phones to enable them to send and receive data more rapidly. With a GPRS connection, the phone is “always on” and can transfer data immediately, and at higher speeds: typically 32-48 kbps. An additional benefit is that data can be transferred at the same time as making a voice call. GPRS is now available on most new phones. GPRS is part of a series of technologies that are designed to move the 2G networks closer to the performance of 3G (the third generation) networks. The key characteristic of a 3G network is its ability to transfer large amounts of data at high speed (up to 2 Mbps), enabling applications like video calling, video downloads, web browsing, email, etc. By increasing the speed of a 2G network, some of these applications become possible, e.g. web browsing and sending or receiving emails with large attachments.
An access point name (APN) is used for establishing packet data connections, and is managed by the MME. Further, the APN is used for identifying a packet data network (PDN) or a service type (e.g. multimedia messaging service (MMS)), wherein the PDN or the service type is carried on the packet data connections.
NAS level congestion control contains the functions: “APN based congestion control” and “General NAS level Mobility Management control”. The use of APN based MM/SM (mobility management/session management) congestion control is for avoiding and handling of MM and SM signalling congestion associated with UEs with a particular APN. With General NAS level mobility management control, the SGSN may also use the reject of NAS level Mobility Management signaling requests under general congestion conditions.
The SGSN may store a mobility management (MM) back-off time and a session management (SM) back-off time when congestion control is active for general NAS level MM congestion control or APN based SM/MM congestion control. The SGSN may enforce the stored back-off time by immediately rejecting any subsequent request from the UE targeting at connecting to the APN before the stored back-off time is expired.
A network node will reject the MS with GMM (GPRS Mobility Management) back-off timer when the network node is applying NAS level congestion control either due to congestion in network being caused due to particular APN or due to General NAS level congestion in the network. The MS during the back off time shall not initiate GMM signalling besides the emergency procedure.
A MS camped normally on a cell will perform the normal attach procedure to get emergency bearer services i.e. a MS will send ATTACH REQUEST message with attach type “GPRS attach” or combined attach depending on network mode of operation (NMO) of the cell. If a MS whose attach procedure is rejected by the network node with GMM back off timer when the network node is applying General NAS level congestion control initiates the emergency attach procedure on normal cell on sending ATTACH REQUEST message with attach type “GPRS attach” or combined GPRS/IMSI attach then the network node will reject the attach procedure again when the network node is running GMM back-off timer and it is not expired. Then the MS will not able to get the emergency bearer services till the back-off timer expires.
In addition, a MS will perform the combined attach procedure for packet switched/circuit switched (PS/CS) services when the network mode of operation (NMO) is NMOI or the MS is configured to use the extended NMO I system information and extended NMO I system information indicates NMO I.
When the MS attached for combined CS/PS services detaches from IMSI services only then it will send detach request message with detach type “IMSI detach”. During this time if the network node is applying the NAS level congestion control then the network node will not able to send GMM back-off timer is detach accept message as detach accept message currently doesn't contain the back-off timer IE. The MS will be rejected in next GMM signalling message with back-off timer.